This invention generally relates to video signal processing apparatus, and more particularly to a signal processing system for stabilizing a television picture reproduced from video produced by a camera subjected to motion.
In the electronic news gathering (ENG) application of television, in which a portable camera and recorder are taken into the field, often the cameraman is subjected to pushing and jostling with the consequence that the picture reproduced from the recorded video output of the camera exhibits an annoying jumpiness which seriously detracts from the viewability of the picture. That is, even objects in the recorded scene normally expected to remain stationary, such as buildings, trees, and the like, move up and down in a jittery fashion as a result of sudden motion of the camera. The effect is particularly annoying when program segments shot with a remote minicam subjected to jostling are interspersed with truly stationary segments shot with a stable camera. The viewability of the reproduced picture can be similarly impaired when a camera equipped with a telephoto lens, even if stably held, is used to record a distant scene, such as a person standing on a platform delivering a speech, since even very slight movement of the outer end of the lens, usually up and down, is magnified and causes the image of the speaker to move up and down in the reproduced picture. Undesirable jitter in the television picture can also occur when the video signal is produced by a camera carried on moving platform, such as a helicopter, particularly when a telephoto lens is used because, again, vibration of the lens is magnified and causes a "jumpy" picture.
Devices are commercially available for stabilizing the image of a video signal produced by a camera carried on an unstable, usually moving platform, which have been used with success in the shooting of television pictures on aircraft, both fixed wing and helicopters. One such device, which operates on opto-mechanical principles, is the image stabilizer distributed by the Arriflex Corporation, Blauvelt, N.Y., which includes a support plate on which the camera is mounted and a stabilizer positioned in front of the camera lens. Entering light rays are reflected off a front-surface mirror mounted on two gimbals powered by a battery-driven gyroscope, and the image from this mirror is reflected onto another fixed mirror and into the camera lens. The directional stability of the gyroscope resists off-axis movement--such as panning of the camera--and a precession brake causes the gyro to lean steadily with the panning motion, thereby to stabilize the image. This type of stabilizer is a real-time device in the sense that it can be used only during actual shooting of the picture. It is not readily adaptable for use with portable television cameras, and even if it could be so adapted the added weight would be inconsistent with the emphasis, particularly for ENG applications, toward smaller and lighter weight cameras. It not being conveniently possible to effect image stabilization during shooting of a scene with a portable camera, if there is to be correction for the jumpiness of the reproduced image caused by jostling or other motion of the camera, it must be later applied to the recorded camera signal, before editing and/or reproduction for broadcasting or viewing.
The described jumpiness in the television picture due to erratic camera motion is different from and is not to be confused with the picture jitter sometimes caused by time base errors introduced during reproduction of video signals recorded on magnetic tape, for example, by reason of expansion or contraction of the tape during or after recording, variations between the tape recording speed and the tape reproducing speed, and the like. The source and consequence of such time base errors will be appreciated from a brief description of the NTSC television system which uses an interlaced scanning system consisting of repeating frames having a first, or odd, field which reproduces alternate lines of each frame and a second, or even, field interlaced with the first field which reproduces the remaining lines of each frame. The odd field consists of parallel lines, the first of which begins at the upper left corner of the television screen and the last of which ends in the middle of the bottom of the screen. The even field consists of parallel lines interlaced between the lines of the odd field, the first of which begins in the middle of the top of the television screen and the last of which ends at the lower right corner of the television screen. The video at a point in a line from an even field, for example, contains video data which reproduces a part of the picture occurring just above or just below a corresponding point on the next adjacent line from the odd field. The vertical distance between such corresponding points is one-half pitch, or one-half the center-to-center distance between adjacent field lines. During the scanning process for each frame, it is important that each succeeding line start and stop at the same horizontal position as the line preceding it so that the vertical edges of the generally rectangular picture are straight and so that the reproduced picture is a true image of the scene being televised. To synchronize the scan of the cathode ray tube with the transmitted video signal a horizontal synchronizing pulse is provided at the beginning of each line, and a vertical synchronizing pulse is provided at the beginning of each field.
Apparatus has long been known for removing time base errors from video signals. Early forms of time base error correctors, suitable for use with video tape recorders having four rotary heads and typically have time base errors of one microsecond or less, employed tapped delay lines, or other forms of delay elements, for variably delaying the incoming video signals in order to compensate for time base errors. Time base correctors in current use typically employ digital techniques to accommodate and correct time base errors of several television lines, as are frequently encountered in currently used popularly priced helical scan video tape recorders. Whatever the type or implementation, all time base corrects operate on the general principle of examining the horizontal sync pulse of the reproduced video signal and comparing it with a stable local reference and then advancing or retarding the reproduced signal so that its horizontal sync pulses are always in synchronism with the local reference. Thus, in a time base corrected video signal, any object within the scene represented by the signal bears a fixed time relationship to the horizontal sync pulse of the television line or lines in which the object appears.
In a video signal produced by a television camera subjected to erratic motion during shooting of the picture the situation is quite different; in this case even though the horizontal sync pulses generated at the beginning of each line are absolutely stable, objects in the scene that are expected to be stationary move around in jumpy fashion, producing an effect just as annoying, or perhaps more so, as picture motion caused by sync instability. In other words, here the horizontal sync is perfectly stable, and the jumpy motion in the reproduced picture is caused solely by camera motion.